1. Field of the Invention
The present invention relates to a disk playing apparatus, and more particularly to a disk playing apparatus for playing a disk on which a digital signal treated by a predetermined modulation process is recorded.
2. Description of Background Information
Information recording systems using a disk-like recording medium are generally classified into CAV (constant angular velocity) and CLV (constant linear velocity) systems. A disk of the CLV system which will be hereinafter referred simply to as the CLV disk has an advantage such that it can record about two times as much information as with a disk of the CAV recording system. Therefore, the CLV recording system is adopted for the digital audio disk called CD (compact disk) and the CD-ROM used as a digital data recording medium, having the same basic recording format.
With such recording disks, a surface recording density can be improved by raising a linear recording density. However, when the linear recording density is improved an interference to the reproduced signal is generated by precedent and subsequent recorded patterns, causing a phase shift, which is called a pattern peak shift phenomenon. In order to reduce the amount of such a pattern peak shift, RLL (Run Length Limited) modulation code format is used in recording digital signal on an optical disk as typically in the recording of the CD.
However, even by using the RLL modulation code format, it is impossible to eliminate the pattern peak shift completely. Accordingly, in a disk playing apparatus, an RF compensating circuit for a read RF (high frequency) signal from the disk is provided so that phase and frequency characteristics of the RF signal are compensated by the RF compensating circuit thereby making a group delay characteristic flat and preventing an increase in data detection error rate. FIG. 1 shows a compensation characteristic of such an RF compensating circuit. As shown in FIG. 1, the RF signal is emphasized by 6 through 10 dB for 3T with respect to 11T, where T denotes a bit period of the pulse train signal. As shown in FIG. 2, a waveform after compensation (dashed line) by the RF compensating circuit can be obtained in comparison with a waveform before the compensation (solid line).
Meanwhile, techniques for the high-speed access of the CD-ROM have been advanced in recent years, in order to satisfy a need of quickly reading desired data from a CD-ROM. However, even if the speed of access is merely increased, still a relatively long time is required for reading a large amount of data such as image information data. Further, there is a limit in reducing a total period of time required for reading the data. Thus, it is necessary to increase a reading speed of recorded information from the CD-ROM. Such a requirement can be met by raising the linear velocity to a value higher than a normal linear velocity, e.g., a value twice or four times the normal linear velocity in reading recorded information.
However, in reading the recorded information at a linear velocity twice or four times the normal linear velocity for example, to increase the reading speed of the data from the CD-ROM, a flat group delay characteristic cannot be obtained unless the RF compensation for the RF signal read from the disk is carried out depending of the higher linear velocity. The variation in the group delay characteristic in turn causes an increase in the data detection error rate.